Cohort 8 (non-ambulatory participants) is currently enrolling new participants. Enrollment for Cohorts 1 through 7 has been completed.

This is an open-label gene transfer therapy study evaluating the safety of and expression from delandistrogene moxeparvovec in participants with Duchenne Muscular Dystrophy (DMD). The maximum participant duration for this study is 156 weeks.

Inclusion Criteria:

• For Cohorts 1-8: Has a definitive diagnosis of DMD based on documented clinical findings and prior genetic testing.
• Cohort 8: Non-ambulatory per protocol-specified criteria at the time of Screening, has a performance upper limb (PUL) entry item score ≥3 at the Screening visit and has a total PUL score of ≥20 and ≤40 at the time of Screening.
• Cohorts 1, 2, 3, 5, 7 and 8 only: Stable dose equivalent of oral glucocorticoids for at least 12 weeks before screening and the dose is expected to remain constant (except for modifications to accommodate changes in weight) throughout the first year of the study.
• Cohort 1: Is ambulatory, and ≥4 to <8 years of age at the time of Screening.
• Cohort 2: Is ambulatory, and ≥8 to <18 years of age at the time of Screening.
• Cohort 3: Non-ambulatory per protocol specified criteria at the time of Screening.
• Cohort 4: Is ambulatory and ≥3 to <4 years of age at the time of Screening.
• Cohort 5a: Is ambulatory and ≥4 to <9 years of age with time to rise from the floor ≤7 seconds at the screening visit.
• Cohort 5b: Non-ambulatory per protocol specified criteria at the time of Screening.
• Cohort 6: Is ambulatory, and ≥2 to <3 years of age at the time of Screening.
• Cohort 7: Non-ambulatory per protocol-specified criteria at the time of Screening.
• Cohorts 4 and 6: Do not yet require use of chronic steroids for treatment of their DMD, in the opinion of the Investigator, and are not receiving steroids at the time of Screening.
• Genetic mutation inclusion criteria vary by cohort.

All Cohorts:

• Ability to cooperate with motor assessment testing.
• rAAVrh74 antibody titers are not elevated as per protocol-specified requirements.

Exclusion Criteria:

• Cohort 8: Any confounding factors that would prevent the use of oral sirolimus including a known hypersensitivity to sirolimus or any of its excipients.
• Has a concomitant illness, autoimmune disease, chronic drug treatment, and/or cognitive delay/impairment that in the opinion of the Investigator creates unnecessary risks for gene transfer.
• Exposure to gene therapy, investigational medication, or any treatment designed to increase dystrophin expression within protocol-specified time limits.
• Abnormality in protocol-specified diagnostic evaluations or laboratory tests.

Note: Other inclusion/exclusion criteria apply.